1. Field of the Invention
The present invention relates to an optical scanner using a laser beam.
2. Description of the Related Art
An optical scanner is known as a device for writing information by scanning a beam of light and is used in a laser printer, a laser facsimile, a laser engraving device, a digital copying machine, etc.
In general, a means for deflecting the light beam by the optical scanner such as a rotary polygon mirror has a so-called deflecting face inclination. To correct this inclination, there is a method in which an image forming lens is arranged between a deflection-reflecting face and a scanned face and constitutes an anamorphic optical system and the positions of the deflection-reflecting face and the scanned face are set in an approximately conjugate relation in geometrical optics with respect to a cross-scanning direction. However, in this method, the image forming lens has refracting powers different from each other with respect to the main scanning direction and the cross scanning direction. Accordingly, it is difficult to preferably correct field curvatures simultaneously in the main and cross scanning directions. Further, when the field curvature in the main scanning direction is preferably corrected, a large field curvature tends to be caused in the cross-scanning direction.
When the large field curvature is caused in the cross-scanning direction, the diameter of a formed image spot on the scanned face in the cross-scanning direction is changed by a scanning position and this change becomes a great obstacle to realization of the optical scanning operation of high density.
In the optical scanner, a special image forming lens system such as an f.theta. lens, etc. is generally used to perform the optical scanning operation at an equal speed. Such an image forming lens system has special optical characteristics so that it is difficult to reduce manufacturing cost and therefore reduce the cost of the optical scanner.
On the other hand, the optical scanning operation can be performed at an uniform speed by electrically correcting an image clock signal for synchronization of modulation of the light beam without moving the light beam at an equal speed. From this viewpoint, optical scanners without using the special image forming lens system such as an f.theta. lens, etc. have been recently proposed. An optical scanner of a post-objective type is a desirable one of such optical scanners and is shown in Japanese Patent Application Laying Open (KOKAI) No. 60-133414.
In a light-deflector of this post-objective type, it is not necessary to dispose the f.theta. lens system and the construction of the deflector is simplified and field curvature is approximately corrected completely with respect to the main scanning direction. The field curvature in the cross-scanning direction is 2 to 5 mm when the optical scanning width corresponds to size A4. There is no problem about the field curvature having such a length when write density by the optical scanning operation is about 400 dpi. However, when the writing operation of high density is performed and the optical scanning width is increased, no diameter of a light spot is constantly stabilized so that quality of printed characters is reduced.